exp
Full Member level 1
The typical impedance used in RF is 50 Ohm and if I build an RFIC with a "RFIN" pin I can expect its source impedance to be 50 Ohm.
However, how does the situation look like in a differential setup, when I have, say RFIN+ and RFIN-?
50 Ohm Single Ended would be 25 Ohm differential.
Does that mean that the source impedance of the RFIN+ pin can be assumed 25 Ohm or would it still be 50 Ohm? (which would mean that externally it would sum up to 100 Ohm).
I know this depends on the application but I am interested in the most common case. Take for example the AD9371 ("Integrated Dual RF Transceiver With Obs Path"). It has differential RF input, RX1+ and RX1-. I would assume that the signal source has 50 Ohm single ended and after the balun this becomes 25 Ohm so that each RX1+/RX1- "sees" 25 Ohm source.
Now the datasheet is more confusing to me. It states: "Input impedance: 200 Ohm Differential". What does that mean? This would imply that the input to each RX1+/RX1- pin is 100 Ohm? And to achieve matching, the source impedance would have to be 200 Ohm, 4x than the common 50 Ohm. Why is this the case?
However, how does the situation look like in a differential setup, when I have, say RFIN+ and RFIN-?
50 Ohm Single Ended would be 25 Ohm differential.
Does that mean that the source impedance of the RFIN+ pin can be assumed 25 Ohm or would it still be 50 Ohm? (which would mean that externally it would sum up to 100 Ohm).
I know this depends on the application but I am interested in the most common case. Take for example the AD9371 ("Integrated Dual RF Transceiver With Obs Path"). It has differential RF input, RX1+ and RX1-. I would assume that the signal source has 50 Ohm single ended and after the balun this becomes 25 Ohm so that each RX1+/RX1- "sees" 25 Ohm source.
Now the datasheet is more confusing to me. It states: "Input impedance: 200 Ohm Differential". What does that mean? This would imply that the input to each RX1+/RX1- pin is 100 Ohm? And to achieve matching, the source impedance would have to be 200 Ohm, 4x than the common 50 Ohm. Why is this the case?